What drives isoprene in tropical Australia?

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Abstract

Isoprene is the dominant biogenic svolatile organic compound (BVOC) emitted
from plants across the globe, with a mass of 400-600 Tg emitted annually. Its
emission and chemical degradation plays a central role in the atmosphere, contributing
to the formation of ground-level ozone and secondary organic aerosol.
Tropical ecosystems contribute up to 75% of the global isoprene budget.
Seasonal isoprene emission patterns in tropical regions remains unclear, particularly
when compared to the mid-latitudes. It was hypothesised that in tropical
regions, isoprene would be consistent throughout the year. However, a 12-year
record of satellite observations of formaldehyde (HCHO) over the Amazon basin
showed that HCHO columns reduced by 20-40% each year during the wet-to-dry
transition. This thesis verifies these observations and investigates the hypothesis
with a long-term, ground-based measurement study in a rainforest environment,
paying particular attention to this transition period.
To improve understanding of seasonal isoprene emissions patterns I conducted
a measurement and modeling campaign in Far North Queensland to understand
the drivers of isoprene emission in tropical Australia. A Fast Isoprene
Sensor was installed in the Daintree Rainforest and I measured canopy-level isoprene
concentrations over three years. They show that isoprene emissions follow
a seasonal cycle, which differs from the Amazon. The measurements are compared
against GOME-2B satellite observations and MEGAN and MLC-CHEM
models to investigate factors driving emission patterns across several timescales.
Findings show that model bias decreases by over 10% when leaf area index varies
in response to the growing season and the emission factor is optimised by time
and season.
This is the first major BVOC study to be conducted in the Daintree, and
is the first isoprene study in tropical Australia in over 20 years. The results
presented in this thesis represent the first observations of seasonal isoprene emission
patterns in Australia and provide an important contrast to other tropical
ecosystems.